The particular playback systems in question are sound playback systems of multi-channel type (5.1, 7.1, 10.2, 22.2, etc.) or ambisonic type (ambisonics in the literature or higher order ambisonics (HOA)).
To allow good quality playback of multi-channel signals, present-day devices for calibrating the acoustics of the listening site are based on a general method of “multi-channel equalization” type in which the impulse responses of each loudspeaker in the playback system are measured using one or more microphones at one or more points at the listening site and frequency equalization filtering is carried out on each loudspeaker, independently, by inverting all or part of the impulse response measured for the loudspeaker in question.
The inversion aims to correct the response of the loudspeaker in such a way that said response comes as close as possible to a “target” curve generally defined in the frequency domain in order to improve the delivery of the tone of the sound sources.
Such a method is described in the document titled “Digital Filter Design for Inversion Problems in Sound Reproduction”, by Kirkeby and Nelson, in JAES 7/8, pp. 583-595, 1999, for example.
This type of calibration or correction focuses on correction of the frequency aspect of the response of the playback system at the listening site without making use of temporal information such as reflection phenomena and notably early reflections of the sound signals.
However, early reflections of sound signals have a non-negligible effect on the auditory perception of the reproduced sound signal.
In addition, the analysis of the impulse responses carried out in existing calibration methods is of monophonic type, i.e. it does not take into account the spatial information of the reflections, such as the direction of incidence, either.
The absence of temporal and spatial data for the reflections does not allow consideration of the role of these reflections in the perception of the direct wave of the sound signal by a listener, and thus adjustment of the correction according to their specific effect. The quality of the sound signal played back and perceived by the listener is then less than optimum.
The techniques of the prior art are based on the application of corrective filters to each of the channels of the multi-channel signal, i.e. each loudspeaker in the playback system is corrected individually without taking into account the whole array of loudspeakers.
There is therefore a need to optimize the calibration carried out on systems for playing back multi-channel audio signals, firstly to take into account the temporal and spatial properties of the sound reflections that affect the auditory perception of the direct waves, in order to adjust the processing endeavor according to the perceptibility of degradation and thus to limit the audible artefacts liable to be generated by the excessively constrained processing carried out in existing calibration methods, and secondly to use the various loudspeakers jointly, in order to distribute the processing endeavor between all the loudspeakers.